As David states correctly, denying the holocaust is illegal in Germany (... a law which starts to become controversial even here where most people are not free speech absolutists. I personally believe in counter speech when it comes to bad ideas and I think bans can be counterproductive, even if they can easily be explained historically and come from a good place).
However, critical reporting about holocaust deniers is not only perfectly legal in Germany, but also completely accepted in German society and mainstream media.
If YouTube was trying to respect German laws or sensibilities in this case, they simply got it wrong.
Yeah, I don't think what took place actually had any connection to laws in the countries where the video was blocked.
Another explanation, at least for Germany, could be the "Netzwerkdurchsetzungsgesetz", a law that basically requires all social media to ban "obviously illegal" content in 24 hours or less or these platforms paying a high fine in the hundredthousands or even millions of € if I'm not wrong.
Because YouTube, Facebook, Twitter, etc. are very big it's not possible to check all content posted on them manually by employees. So all these platforms are either using an algorithm-based mechanism (f.e. YouTube, as is very well known and infamous) or a frequently abused user-based system via the report-function (f.e. Twitter) and therefore this law leads to overblocking. The german satire-magazine "Titanic" even was one of the first victims of this "Netzwerkdurchsetzungsgesetz".
So the reason behind a video-ban in Germany could at the end be just mentioning the word "holocaust" in the title as @davidpakman did and the YouTube-algorithm overreacting to it to make sure that YouTube doesn't need to pay a high fine but I'm not sure about possible reasons in other countries like UK.
They don't really care about our laws, they care about legal costs. When the price of a Lawyer exceeds the profit potential, they will block the video. In the U.S. search engines like Google and YouTube are protected by law from liability for what others post. In Europe that isn't always the case and they could be liable as was proven in a 2015 case commonly known as the right to be forgotten. https://en.wikipedia.org/wiki/Right_to_be_forgotten
I think it’s a result of YouTube’s algorithm automatically banning/hiding halocaust videos in certain countries/areas. Not a legality thing.
It's also illegal to argue that the scale of the holocaust was less than is reported... which is a platform of the guy... now that I think about it I bet that's it.